This application claims a priority based on German patent application 199 22 440.4, filed May 6, 1999, and the contents of that priority application are incorporated herein by reference.
The invention involves a pedicle screw having a threaded screw shaft and a screw head, with the screw head having mechanisms for affixing correction elements.
Pedicle screws are known in many diverse forms. As a rule, a pedicle screw has a screw shaft, whose length and diameter are adapted to a bone diameter and which is threaded so that the pedicle screw can be screwed into the bone in a problem-free and secure manner and, above all, without destroying the bone. The screw head is structured such that correction and/or fixing tabs can be mounted on it in a play-free manner, for achieving osteosynthesis.
It has been shown that for all pedicle screws there is a danger, particularly for persons with osteoporosis, that, although the screws can be screwed into the bone in a problem-free manner, the screws, however, either loosen by themselves or are sometimes even be pulled out of the bone by application of large external forces.
It is thus an object of this invention to provide a pedicle screw of the type mentioned in the opening paragraph above that can be securely anchored in bone and which considerably reduces the danger of its being pulled out.
According to principles of the invention a free end of a screw shaft of a pedicle screw, of the type mentioned in the opening paragraph above, is equipped with a pull-out prevention device; in particular with a holding device, which is independent of a first threading, for holding a safety mechanism that secures the pedicle screw against unintentional detachment after it is screwed in.
Due to the safety mechanism, which is attached to the free end of the threaded shaft of the inventive pedicle screw once the screw has assumed its final position in the bone, a danger of its being pulled out of the bone is considerably reduced since the pedicle screw is not only supported on the bone, especially on a superior thoracic vertebrae, by threading, but also by the safety mechanism. A side of the safety mechanism lying opposite the screw head rests on the bone and the bone is held between the screw head and the safety mechanism.
In an enhanced embodiment the holding device defines at least one circumferential groove. The safety mechanism is supported in this circumferential groove on the screw shaft and is held securely to the screw shaft in the circumferential groove. The circumferential groove also has the advantage that the safety mechanism can be affixed to the shaft about the entire circumference of the shaft.
An additional enhancement provides that the holding device defines an additional threading. This threading can have a different spacing and/or pitch than the screw-in threading of the screw shaft. As a rule, this additional threading is a machine threading, so that standard parts or safety mechanisms having standardized inner threading can be engaged therewith. In addition, threads of this type are relatively easy to manufacture.
A cost-effective embodiment of the safety mechanism is, for example, a locking washer, which can be manufactured as a stamped part or as a stamped bent part. A locking washer has, in addition, the advantage that it has a large area for resting on the outer surface of the bone, and, in this way, a holding force is optimally distributed.
In a preferred embodiment, the locking washer has an opening for receiving the holding device. A free end of the screw shaft is held in this opening, with dimensions of the opening being selected so that an edge defining the opening lies in the circumferential groove, for example, or in the additional threaded groove.
A simple attachment of the safety mechanism is achieved in that the opening is also open at its edge. In this regard, an edge-opening slit of the opening can have a slightly smaller clearance width than a diameter of the free end of the screw shaft.
In order to be able to optimally oppose load peaks, the safety mechanism is structured to be elastic at a connection area where it is connected with the holding device. In this way, a rigid construction that is susceptible to failure is avoided and the elastic embodiment additionally has the advantage that after overcoming load peaks, the safety mechanism assumes its starting, or rest, position again, by springing back elastically.
In this regard, in a preferred embodiment, the elastic structure can be a bend. For this, the safety mechanism is preferably manufactured from a spring steel.
A preferred additional embodiment provides that the safety mechanism can be clipped, or snapped, onto the holding device. This embodiment allows the safety mechanism to be mounted on the free end of the screw shaft, after the pedicle screw is screwed in, in a relatively simple manner by merely being pressed, or forced, on, such that it preferably self engages with, or snaps onto, the free end of the screw shaft.
In order to achieve a specific elasticity of the safety mechanism, it has one or more slits. Between the slits, bulges, or wings, are provided, which extend, or protrude, radially and have free ends lying on the screw shaft.
An additional embodiment provides that the safety mechanism has openings or holes in which a tool can engage. Using this tool, the safety mechanism can be attached and/or removed. Preferably, a screwing movement is done for accomplishing this.
Preferred materials for the safety mechanism are metals, in particular, spring steel, stainless steel, or titanium, or the safety mechanism is made of resinous plastic or a composite material.